1. Field of the Invention
The present invention relates to spacer grid spring that increases the conformal contact area with a fuel rod and, more particularly, to spacer grid spring that increases the conformal contact area with a fuel rod, in which the spacer grid spring of the grid strap, which is in contact with the fuel rod, is optimized in shape, thereby the elasticity of the spacer grid spring contacting the fuel rod is increased, the conformal contact area with a contact portion of the spacer grid spring contacting the fuel rod when the fuel rod is inserted into each cell of a spacer grid spring is expanded to realize uniform stress distribution, and excessive plastic deformation of the spacer grid spring can be reduced, and in which magnitude and distribution of contact pressure between the fuel rod and the spacer grid spring are improved, thereby reducing the possibility of fretting wear caused by contact between the fuel rod and the spacer grid spring.
2. Description of the Related Art
In general, spacer grids is one of the components constituting a nuclear fuel assembly in a nuclear reactor, and functions to arrange and support nuclear fuel rods, which constitute unit grid cells thereof, at designated positions by mutually connecting a plurality of straps, each of which has a spacer grid spring and dimples.
FIG. 1 is a perspective view schematically illustrating a conventional nuclear fuel assembly. FIG. 2 is a plan view schematically illustrating spacer grids applied to a conventional nuclear fuel assembly. FIG. 3 is a perspective view schematically illustrating spacer grids applied to a conventional nuclear fuel assembly. FIG. 4 is a perspective view schematically illustrating spacer grid spring for spacer grids applied to a conventional nuclear fuel assembly. FIG. 5 schematically illustrates deformation of spacer grid spring, which is applied to spacer grids of a conventional nuclear fuel assembly, caused by one nuclear fuel rod.
As illustrated in the figures, the conventional nuclear fuel assembly 2 to which spacer grids 110 are applied has a plurality of guide tubes 113 fitted between a top end piece 111 and a bottom end piece 112.
Here, the spacer grids 110 supporting nuclear fuel rods 125 form the nuclear fuel assembly 2 by means of welding with the guide tubes 113 in the longitudinal direction of the guide tubes 113 at regular intervals.
Meanwhile, the spacer grids 110 is usually formed of zircaloy, and includes nuclear fuel rod cells 123 that support the nuclear fuel rods 125, and guide tube cells 124 into which the guide tubes 113 are inserted, wherein each nuclear fuel rod cell 123 is designed such that two spacer grid springs 118 and the total of four dimples 119, which are each located on a top side and a bottom side of each spacer grid spring 118, contact and support each nuclear fuel rod 125 at six supporting points.
Thus, when the spacer grid springs 118 are in contact with the nuclear fuel rods 125 and are deformed by the nuclear fuel rods 125, two supports 121 of each spacer grid spring 118 are pressed by each nuclear fuel rod 125. Here, because the rigidity of each support 121 is similar to that of a central curvature 122, which is connected with the two supports 121 and is contact with each nuclear fuel rod 125, each support 121 is simultaneously subjected to bending 131 and twisting 132 due to a load 130 applied by each nuclear fuel rod 125, as illustrated by arrows in FIG. 5.
When these bending and twisting deformations act on each support at the same time, each nuclear fuel rod 125 is unstably supported, and thus slight sliding occurs between the nuclear fuel rod 125 and the central curvature 122 contacting the nuclear fuel rod 125. As a result, desired conformal surface contact does not occur, thus causing stress concentration. This stress concentration makes the spacer grid spring vulnerable to fatigue, so that each nuclear fuel rod 125 has a high possibility of increasing the fretting wear thereof.
Further, the deformation of the central curvature 122 provides a high possibility of losing the original surface contact profile, and thus exerts an influence on stress distribution and magnitude of peak stress.
Meanwhile, the spacer grid spring 118 and the dimples 119 of each grid strap 115 have the same radius of curvature as the nuclear fuel rods 125, and thus are in conformal surface contact with the nuclear fuel rods 125 from the beginning. Thereafter, when the spacer grid spring 118 receives the load of each nuclear fuel rod 125, the central curvature 122 contacting respective nuclear fuel rod 125, and its support 121 are deformed at the same time.
Further, when spring force of the spacer grid spring 118 and dimples 119 of each grid strap 115 is too weak, the nuclear fuel rods 125 cannot be arranged at designated positions, and thereby have a possibility of losing sound supporting performance. When the spring force is too high, each nuclear fuel rod 125 undergoes defects such as scratching on the surface thereof due to excessive frictional resistance when inserted into the spacer grid 110. Further, during operation of the nuclear reactor, the nuclear fuel rods 125 experience longitudinal growth by means of irradiation of neutrons. This longitudinal growth is not properly accepted, and thus the nuclear fuel rods 125 are bent.
In this manner, when the nuclear fuel rods are bent, the neighboring nuclear fuel rods become too near each other or contact each other. Thus, the coolant channel between the neighboring nuclear fuel rods becomes narrow or is blocked. As a result, heat generated from the nuclear fuel rods is not effectively transmitted to the coolant, thereby increasing the temperature of the nuclear fuel rods. As such, the possibility of generating departure from nucleate boiling (DNB) is increased, which is mainly responsible for the reduction of nuclear fuel output.
In order to solve the above-described problems, recent nuclear fuel development has focused on high combustion and zero defects. In particular, to develop high-combustion nuclear fuel, the thermal performance of the nuclear fuel for promoting heat transmission from the nuclear fuel rods to the coolant must be increased. To this end, a method of improving the flow of the coolant flowing around the nuclear fuel rods is required.
Here, as the method of improving the flow of the coolant flowing around the nuclear fuel rods, a method of changing the geometry of the spacer grid is employed, and may include the attachment of a hybrid vane or a change in its design, or effective construction of a fluid channel.
However, most concepts for raising this thermal performance are based on the principle that the flow of the coolant flowing around the nuclear fuel rods is very turbulent, the flow has a high Reynolds number. In this manner, the turbulence of the coolant flow around the nuclear fuel rods is mainly responsible for, flow induced vibration of the nuclear fuel rods.
The flow induced vibration of the nuclear fuel rods is a factor generating mutual relative motion, in which the nuclear fuel rods slide on the contact surfaces with the spacer grid springs or the dimples of the grid straps. For this reason, the contact surfaces of the nuclear fuel rods are subjected to local wear, which incurs “fretting damage to the nuclear fuel rods”, in which the nuclear fuel rods are gradually damaged.
In other words, the contact surfaces between the nuclear fuel rods and the spacer grid springs or dimples of the grid straps are worn, so that the nuclear fuel rods are locally damaged. When this damage becomes serious, the nuclear fuel rods can be broken.
Therefore, the method of raising the thermal performance of the nuclear fuel in order to develop high-combustion nuclear fuel leads instead in damage to the nuclear fuel rods.
As described above, the spacer grids serving to support the nuclear fuel rods must be able to maintain reliable supporting performance during the lifetime of the nuclear fuel rods, and to inhibit the possibility of fretting wear of the nuclear fuel rods.
In this manner, in order to allow the nuclear fuel rods to maintain reliable supporting performance for the lifetime of the nuclear fuel rods, the spacer grid springs must be able to support the nuclear fuel rods with sufficient spring force for the lifetime of the nuclear fuel rods, and to maintain at least enough spring force to support the nuclear fuel rods, which can be variously changed in the nuclear reactor up to the lifetime of the nuclear fuel rods, by expanding the elastic behavior region of the spacer grid springs.
However, during the operation of the nuclear reactor, the spacer grid springs and dimples gradually lose the initial spring force applied to the nuclear fuel rods due to the irradiation of the neutrons. As a result, a gap can develop between the nuclear fuel rods and their supports, and reliable supporting performance of the nuclear fuel rods can be lost by means of the load acting on the nuclear fuel rods in an arbitrary direction due to the flow of the coolant.
Further, in order to inhibit the possibility of the fretting wear of the nuclear fuel rods, the causes of fretting wear must be reduced. These causes generate the gap between the nuclear fuel rods and the supports of the spacer grids due to reduction of the spring force caused by the neutron irradiation, thermal expansion difference between the nuclear fuel rods and the spacer grids, diametrical reduction of the nuclear fuel rods caused by the elongation of the nuclear fuel rods, and so on. The nuclear fuel rods are vibrated by the turbulent flow caused by the coolant flow, and thus fretting wear is accelerated.